Gun shop near post office OK?

A friend is about to open a gun shop accross the street from the local Federal Post Office and someone has filed a complaint with the city, in an effort to stop the business, stating that it was illegal for a firearms shop to be with in 1,000 feet of a federal building.
We can find no such regulation on the books to back this complaint up. Is anyone aware of such a law? If so, please advise the details.

TOTAL BS!
I know a gun shop that is in the very SAME building as a Post Office.
And it's also located in the most left-wing, sozialist, anti-gun city in The Peoples Republik of Massachusetts. Roaches Sporting Goods, the ONLY gun shop in Cambridge. Me and my buddies even joked about it being in the same building as a Post Office. We'd tell people that they had a back counter with a with a pass through window between them so the postal employees could easily place their orders........